Lighting devices of this kind are generally provided with a high-voltage pulse generating circuit that applies a high-voltage pulse to a high-pressure discharge lamp to start the lamp. One example of the lighting devices is depicted in FIG. 5. In this figure, reference numeral 1 denotes an AC power source, reference numeral 2 denotes a high-pressure discharge lamp, reference numeral 3 denotes a ballast, reference numeral 4 denotes a high-voltage pulse generating circuit, reference numeral 5 denotes a pulse transformer, reference numeral 6 denotes a capacitor, and reference numeral 7 denotes a switching element. When the switching element 7 turns from an OFF state to an ON state, a pulse-shaped electric current flows through a primary winding N1 of the pulse transformer 5 via the capacitor 6 and, hence, a pulse-shaped high voltage is generated in a secondary winding N2 of the pulse transformer 5. As a result, insulation of the high-pressure discharge lamp 2 is broken to thereby start discharge. When the high-pressure discharge lamp 2 is turned on, the AC power source 1 supplies the high-pressure discharge lamp 2 with electric power via the ballast 3.
It is known that the high-pressure discharge lamp of this type is hard to be turned on again immediately after it has been turned off, because a high lamp temperature increases the gas pressure inside an arc tube. Accordingly, the user must try the starting for, for example, about 20 minutes. If the high-pressure discharge lamp 2 is not turned on, the switching element 7 repeats ON and OFF and continues generating the high-voltage pulse. It is not preferable to continue the application of the high-voltage pulse, because it sometimes causes noise or gives a stress to circuit elements.
Patent document 1 (Japanese Laid-Open Patent Publication No. 6-260289) discloses that upon setting a delay time depending on the lighting duration, the high-voltage pulse is applied to thereby minimize the time of application of the high-voltage pulse.
On the other hand, in the high-pressure discharge lamp of this type, gas leakage from an inner tube (arc tube) sometimes occurs at the end of the life thereof, and the gas collects in an outer tube of the discharge lamp. In such a case, when a high-voltage pulse is applied, abnormal discharge (discharge in the outer tube) occasionally occurs between metallic elements, which support the arc tube, in the outer tube (see FIG. 6). Under this condition, an outer tube glass or a threaded plug (base) of the discharge lamp comes to have a high temperature, causing an energy loss. Furthermore, when the discharge in the outer tube occurs, the temperature of the metallic elements for supporting the arc tube becomes high and exceeds, in some cases, a thermionic critical temperature at which thermoelectrons are emitted and, hence, discharge is likely to occur at such portions. As a result, when a high-voltage pulse is applied, the discharge starts between the metallic elements for supporting the arc tube, causing abnormal discharge or discharge in the outer tube. Because the construction of Patent document 1 referred to above takes no measures against the discharge in the outer tube, if the discharge lamp is supplied with electricity at the end of the life thereof, there is a good chance that discharge occurs in the outer tube.
Half-wave discharge is another abnormal discharge state that may be foreseen at the end of the life of the high-pressure discharge lamp. This is caused by deterioration of an electrode on one side, which proceeds with the age of the high-pressure discharge lamp. Under such a condition, the lamp current flowing through the high-pressure discharge lamp becomes asymmetric on the positive side and on the negative side, and the high-pressure discharge lamp is in a nearly short-circuit condition on one side and in a nearly no-load condition on the other side. In the case of a copper- or iron-based ballast, a direct current flows therethrough and an electric current more than three times the normal secondary short-circuit current flows through one side polarity, causing deterioration of the ballast. A method of adding the ballast with an element such as a thermal fuse or a thermal protector is known as a countermeasure. However, because the thermal fuse is of a non-return type, once the half-wave discharge occurs in the lamp, the ballast can be no longer used, and because the thermal protector is of a return type, the use thereof results in repetition of ON and OFF and is hence not preferable as a countermeasure.
Patent document 2 (Japanese Laid-Open Patent Publication No. 2002-352969) discloses that upon detection of half-wave discharge, the power supply to the high-pressure discharge lamp from the ballast is once cut off, and the stop of operation of an igniter (high-voltage pulse generating circuit) is maintained by a signal from a cutoff detecting means for detecting cutoff. This construction makes it possible to prevent deterioration of the ballast or repetition of ON and OFF of the high-pressure discharge lamp when half-wave discharge has occurred.
Patent document 1: Japanese Laid-Open Patent Publication No. 6-260289
Patent document 2: Japanese Laid-Open Patent Publication No. 2002-352969